C Unix – How to Get Absolute Path of a File

absolutec++filesystemspathunix

How can I convert a relative path to an absolute path in C on Unix?
Is there a convenient system function for this?

On Windows there is a GetFullPathName function that does the job, but I didn't find something similar on Unix…

Best Answer

The realpath() function shall derive, from the pathname pointed to by file_name, an absolute pathname that names the same file, whose resolution does not involve '.', '..', or symbolic links. The generated pathname shall be stored as a null-terminated string, up to a maximum of {PATH_MAX} bytes, in the buffer pointed to by resolved_name.

If resolved_name is a null pointer, the behavior of realpath() is implementation-defined.

The following example generates an absolute pathname for the file identified by the symlinkpath argument. The generated pathname is stored in the actualpath array.

#include <stdlib.h>
...
char *symlinkpath = "/tmp/symlink/file";
char actualpath [PATH_MAX+1];
char *ptr;


ptr = realpath(symlinkpath, actualpath);

Related Solutions

Linux – How to Get Absolute Path from Relative Path Programmatically

As Paul mentioned, use realpath(). Please note though, that since many file systems in Linux support hard links, any given directory can have a number of different absolute paths.

Bash – How to Get the Absolute Directory of a File in Bash

To get the full path use:

readlink -f relative/path/to/file

To get the directory of a file:

dirname relative/path/to/file

You can also combine the two:

dirname $(readlink -f relative/path/to/file)

If readlink -f is not available on your system you can use this^*:

function myreadlink() {
  (
  cd "$(dirname $1)"         # or  cd "${1%/*}"
  echo "$PWD/$(basename $1)" # or  echo "$PWD/${1##*/}"
  )
}

Note that if you only need to move to a directory of a file specified as a relative path, you don't need to know the absolute path, a relative path is perfectly legal, so just use:

cd $(dirname relative/path/to/file)

if you wish to go back (while the script is running) to the original path, use pushd instead of cd, and popd when you are done.

* While myreadlink above is good enough in the context of this question, it has some limitation relative to the readlink tool suggested above. For example it doesn't correctly follow a link to a file with different basename.

Best Answer

Related Solutions

Linux – How to Get Absolute Path from Relative Path Programmatically

Bash – How to Get the Absolute Directory of a File in Bash

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